Known magnetic stimulators comprise generally a charging circuit, a capacitor, a large magnetic coil and a control for allowing discharge of the capacitor through the coil. The ‘stimulating’ coil is usually of a size adapted to fit partly over a human cranium.
The discharge capacitor may be discharged, normally by means of a switch in series between the capacitor and the coil or, in more sophisticated embodiments, by electronic switches such as thyristors which not only allow discharge of the capacitor through the coil but also facilitate the recovery of electrical energy by the capacitor from the stimulating coil. One suitable arrangement for this purpose is described in U.S. Pat. No. 5,766,124 to Polson, commonly assigned herewith. Although it is possible to provide a single discharge of the capacitor through the stimulating coil, more versatile arrangements, such as that described in the aforementioned patent, allow for repeated discharges, at a repetition frequency of typically 100 Hz. The aforementioned patent describes energy recovery systems which facilitate the provision of repeated discharges from the capacitor before another charging cycle is necessary.
In any event, the discharge of the capacitor through the coil produces for the coil a time varying magnetic field which stimulates neuro-muscular tissue. This stimulation has well-established therapeutic effects. It has been known to construct a stimulating coil for the aforementioned purposes as a generally flat circular coil, that is to say with the turns of the coil in generally the same plane or, in some cases, progressively offset planes. Owing to the very high magnetic fields required, typically of the order of currents required, the current density of the coil being typically in excess of 108 amperes per meter squared, a stimulating coil is typically composed of pre-formed rectangular solid copper strips rather than wires, there being comparatively few turns, such as between ten and twenty turns, in the coil.
The discharge circuit of a magnetic stimulator of this type is of necessity a resonant LC circuit dominated by the capacitance of the discharge capacitor and the inductance of the stimulating coil. The natural resonant frequency of such a circuit is typically substantially above the aforementioned repetition frequency and is generally in the range from 2 to 6 kHz. For a resonant circuit of which the discharge capacitor has a capacitance of, typically, 90 μF and a stimulating coil having an inductance of, typically, 22 μH, the naturally resonant frequency is 3.6 kHz.
At this comparatively high frequency, owing to the electrical phenomena known as the skin and proximity effects, there is a very significant non-uniformity in the current density through the solid body of the coil. In effect the current through the coil flows through a much-reduced area, increasing the effective resistance of the coil and dissipating more energy within the coil. Although the skin effect on a single conductor is in itself slight for frequencies in the range (for example) of 1-10 kHz, it has now been found that the proximity effect of high frequency current on adjacent turns of a coil renders the current distribution very non linear and substantially increases the dynamic or high frequency resistance of the coil.
The present invention is aimed at alleviating these disadvantages.
The present invention is based on the provision of a stimulating coil which is composed of a plurality of multiple-turn windings of uniformly solid material disposed face-to-face, i.e. in adjacent parallel planes, each turn in each winding being aligned with and separated from a neighbouring turn of an adjacent winding by a gap which is selected for optimum electrical effectiveness. By ‘electrical effectiveness’ is meant a ratio which relates the stimulating strength, i.e. the voltage calculated to be induced across a typical nerve membrane, to the energy dissipated in the coil during stimulation. This effectiveness will be a maximum when the dynamic resistance of the coil is at a minimum.
As will become apparent the gap's height for maximum electrical effectiveness does vary with a variety of factors, such as the number of turns, the width of the turns, the total height of the coil, the radius of the coil and resonant frequency. Nevertheless, it has been found for preferred coil parameters that maximum effectiveness is achieved with a gap of the order of 18 to 20% of the total coil height (the dimension in the direction normal to the plane of the coil and including the gap).
Further features of the invention will be apparent from the following detailed description, with reference to the drawings.